If a number is provided it decides on which TCP port the builtin HTTP-Server will listen on. If set to zero it will automatically choose a port between 18200 and 18215, trying every single one till it finds an unused port. Make sure that this port is opened in your Firewall / NAT. Since JK2MV 1.1 external HTTP Servers are supported. The URL should point to the GameData directory of your file server. Note that clients also need at least JK2MV 1.1 in case you are using a URL. Older JK2MV versions will not detect the availability of HTTP Downloads in this case.